Y axis is inacurate

Builder

ive cut a few simple things on my ox cnc machine, and I seem to keep getting the same problem, cuts along the X axis seem fine, but the cuts along the x axis are about 2mm out. if I were to cut a square out for example the measurement from the left side to the right would be about 2mm larger than the measurement from the top to the bottom. does anyone know why this could be?

Builder

First, verify that when you cut square you are actually cutting square. If not, then your gantry is misaligned and that could be the problem. You might be ending up with a parallelogram that is correctly sized on one side but off by 2mm on the other side. Square your gantry with a laser square. The laser lines let you accurately align a 90 degree angle even when the two things you are aligning aren't coplanar (like a gantry and rails).

Second, calibrate your steps/mm properly on each axis independently. Don't cut or make a shape and measure the resulting item. Instead, clamp a piece of scrap right next to your gantry plate, jog the axis away from the scrap, then measure the distance you can as precisely as you can. A digital caliper is a good device to have for this. Once you measure the actual distance travelled, you now have a ratio that you can multiply with your current steps/mm configuration number to get the correct answer. And then you should run it again in order to make sure that you did it right and to see if you can get it any more accurate.

Third, make sure that your belts (if you are using belts) are properly tensioned and that you have approximately the same tension on both sides of the Y-axis. If the belts are different tensions, then each side will move a different amount for the same number of stepper revolutions.